Out of these Pokemon, Dragonite and Garchomp are the strongest and most versatile by far. Flygon is a worse Garchomp in every regard except for Dragon Dance, but other Pokemon do it better. Salamence, while strong, lacks the ability to set up Dragon Dance safely in today's generation.
DRAGONITE:
Dragonite has a whole host of advantages including Multiscale from Gen 5 onwards, Roost to complement Multiscale, base 134 Attack, and specific coverage options lacking from Salamence, like Ice coverage from Ice Spinner. Multiscale is a huge advantage, as it is able to set up safely in front of a plethora of Pokemon, even those who threaten super effective damage against it. Additionally, Dragonite has Extreme Speed, allowing it to also be an effective revenge killer for its teammates.
Dragonite mainly suffers from a few issues. First, to fully utilize Multiscale, it has to hold Heavy Duty Boots, meaning it is unable to hold something like Lum Berry for burns of Yache Berry to reduce the power of Ice-type moves. Secondly, Dragon-Flying isn't the best defensively, and with the plethora of faster Dragons and Choice Scarf Pokemon that outspeed even +1 Dragonite, like Scarf Lele, Dragonite may struggle to sweep even in the most ideal of situations.
GARCHOMP:
Garchomp has two main sets it can run: bulky hazard support and Scale Shot Swords Dance. In Generation 9, Garchomp received Spikes, allowing it to either run both Rocks and Spikes or choose one while its teammate covers the other. In addition to hazards, Garchomp has impactful utility moves like phasing with Roar and Dragon Taila and Toxic. Scale Shot Swords Dance is a poor man's Dragon Dance, but thanks to the Loaded Dice item (also introduced in Gen 9), Scale Shot becomes a reliable 100 to 125 base power STAB move (accuracy willing). This means that, after a boost to its Speed and Attack, Garchomp can easily sweep weakened teams, leveraging it's high 102 Speed stat to outspeed even common Choice Scarf Pokemon like the aforementioned Lele. Additionally, since Chomp can choose to boost either its Attack or its Speed, it can, for example, only use Swords Dance in front of bulkier teams that cannot easily threaten Chomp.
Of course, both of Garchomp's sets have flaws. Bulky Chomp suffers from a lack of reliable recovery and is unable to hit back against Fairy types, with its only options being Roar, Toxic, and potentiall Earthquake if that is used. Dragon/Ground is also not the greatest defensive typing, with Ice coverage and Fairy Pokemon around every corner. Offensive Garchomp suffers from requring Scale Shot to pull off its speed boost, meaning that any Fairy can effectively invalidate it from gaining one. The Defense drop also makes it susceptible to be revenge killed by the likes of Rillaboom's Grassy Glide, Scizor's Bullet Punch, and various Sucker Punches.
Overall, I think Dragonite is the better out of the two purely because of Multiscale. Yes, Garchomp can fill some roles that Dragonite cannot, like bulky hazards, but Multiscale allows Dragonite to play defense while posing a significant offensive threat. This is unlike both of Garchomp's sets: the defensive set cannot threaten much damage, while the offensive set cannot live big hits. Dragonite at full HP can do both, and because of this, it is an extremely splashable Dragon Dance Pokemon on many teams.
